The present invention is directed to a caster including an electrostatic discharge arrangement providing an electrically conductive path between the article it supports and the surface on which it is supported thereby preventing sparks due to the build up of static electricity which may interfere with data transmission or storage on computers or damage components or cause an explosion in a volatile environment.
It is known in the art to provide electrical grounding contacts between vehicle bodies and the ground to maintain the vehicle body in electrical contact with the ground surface so that in case the vehicle should be electrically charged, the current will be conducted to the ground surface thus shielding the vehicle. See for example, the arrangements shown in U.S. Pat. Nos. 907,104, 1,999,414 and 2,549,471. In general, these arrangements consist of a dangling metallic chain connected at one end to a vehicle axle with the other end scraping along the ground surface. In at least one instance, the same approach was tried, unsuccessfully, with a caster. The difficulty encountered with trying to adapt the dragging chain method to a caster is that the dragging chain skips along the surface on which the caster rolls and sometimes is caught under the caster wheel and is torn off and often loses contact with the ground sometimes lodging inside the periphery of the caster wheel eliminating the conductive path. Since the links of the chain are connected in series, this presents additional difficulties. For a body ground device, see U.S. Pat. No. 2,712,098.
Some previous casters included enclosures that were intended to reduce the risk of a spark jumping from the caster to the ground. Some casters included antifriction arrangements as a means of avoiding the creation of sparks to some extent.
The most commonly used conductive casters previously used nonmetallic wheels impregnated with carbon, as carbon black or carbon fibre, whereby electrical charges were conducted away from the caster through the wheels themselves. The problem with such an arrangement was that after a period of use, they failed to function because dirt and wax built up on the surfaces of the wheels acting as an insulation, the result being that the conductive feature no longer functioned to draw off the electricity. Additionally, the introduction of carbon into wheel mold cavities is difficult to accomplish without material segregation causing the wheel to shrink unevenly and/or denigrating characteristics of the plastic materials. The use of carbon black can discolor the surface which the wheel contacts and also makes wheel colors other than black difficult to achieve.